1. Field of the Invention
The present invention relates to thermoplastic molding compositions with high low temperature impact strength, molded articles prepared from such molding compositions, and a process for producing such molded articles.
2. Discussion of the Background
Industry places high requirements on the material properties of thermoplastic molding compositions, for example for conveyor pipe lines. With regard to the required quality, they must show inert behavior and must also remain stable in shape in the presence of thermal stress. For lines that are used outdoors, there is the requirement that such composition be able to withstand the effects of weather. Thus, such compositions must not lose their practical properties under the action of solar radiation, humidity, cold, etc.
All conventional polyamides (PA) are generally suitable for molded parts. However, especially for outdoor use, there are limitations to the utility of polyamides. Thus, there are no known industrially used polyamides that have at least one monomer with .ltoreq.10 carbon atoms in the carbon chain that are not highly sensitive to water or atmospheric humidity. The sensitivity to humidity and water results in a sharp decrease of low temperature impact strength.
Polyamides with a larger number of carbon atoms in the carbon chain are more suitable, for example PA 11 or PA 12, since they show improved weathering stability and thus also improved low temperature impact strength. However, these higher polyamides, which are readily available industrially, suffer from the drawback that when plasticizers are used with them, the low temperature impact strength again decreases. The decrease in low temperature impact strength may be counteracted by the use of an impact strength modifier (see: DE-AS No. 27 16 004). With this approach, however, the rigidity and solvent resistance of the molding composition suffer.
PA 11 and PA 12 are used in the automobile sector in the plasticized form as fuel lines and as compressed air brake lines. An important requirement for such lines is that they must pass the pipe impact test according to the American Standard SAE J 844 d at -40.degree. C., with tubes with dimensions of 6.35.times.1 mm being considered to be particularly critical.
In this test, such tubes made of plasticized polyamides may break. The sensitivity of critical tube dimensions to mechanical stress is found to increase with higher extrusion rates, i.e., for example, it is pronounced at extrusion rates above 40 m/min.
Thus, there remains a need for molding compositions based on polyamides which do not exhibit a decrease in low temperature impact strength on exposure to moisture, molded articles produced from such molding compositions, and a process for preparing such molded articles.